In various operations related to the drilling of boreholes in the earth for purposes of production of gas, oil or other products, rotary drilling mechanisms are well known. In the process of controlled-direction drilling, often referred to a Measure While Drilling (MWD), apparatus using magnetometers and accelerometers is used to determine the direction of the borehole. However, if the magnetometers and accelerometers employed in the direction sensing apparatus are in rotation along with the drill string and drill bit, substantial inaccuracy problems result. General practice has been to stop drilling when measurements of borehole attitude are required. In the process of determining borehole inclination and azimuthal direction, from the magnetometer and accelerometer data, it is necessary to transform the measured data into an earth-fixed coordinate set.
Several patents disclose the use of means to compute borehole direction parameters while drill string rotation continues, so that it is not necessary to stop the drilling process to make measurements. Examples of such patents are U.S. Pat. Nos. 4,813,274, 4,894,923, 5,012,412 and 5,128,867. All of these provide means to process the data from the magnetometer and accelerometer sensors in such a manner that the data obtained and related to inclination and azimuthal direction of the borehole can be isolated from the rotary environment.
These prior methods remain sensitive to the dynamics of the rotary motion of the drilling apparatus as drilling progresses. If the drilling continues at a near constant-rotation rate for the drill bit, reasonable results can be obtained. However, if the drill bit undergoes what is known as stick-slip rotary motion, serious errors may be encountered. The stick-slip phenomenon is one in which the drill bit may become stuck in the formation, a large twist may then be built up in the drill string from the bottom hole location of the bit to the surface, and when the bit becomes free the drill string will rapidly-spring back with a very high instantaneous untwisting rotation rate for the downhole assembly that carries the magnetometer and accelerometer sensor. Under such conditions, the prior methods referred to above may lead to substantial error in the desired output information.
U.S. Pat. No. 4,472,884 shows a magnetic survey tool and use of a rotary drive about the borehole axis. However, this tool does not provide any isolation of input angular rates about the borehole axis, and instead uses the rotary drive to make multiple measurements about the borehole axis.
It is a major object of this invention to provide apparatus and method to overcome problems as referred to, through provision of an inertial angular rotation sensor having an axis of sensitivity along the borehole direction to stabilize either the direction of measurement or the resulting data from the magnetometer and accelerometer data provided by the magnetic field and acceleration sensors.
Apparatus provided by the invention includes a set of magnetometers for measuring components of the earth""s magnetic field, a set of accelerometers for measuring components of the earth""s gravity field and an inertial angular rotation sensor having an axis of sensitivity along the direction of the borehole axis. Control, power and processing circuitry is provided to operate these sensing devices and to process the outputs of the sensing devices to obtain stabilized component data in a coordinate system that does not rotate with the the measurement apparatus.
In one embodiment, a rotary drive means is provided to rotate the sensing devices, and about an axis of rotation along the borehole axis. Such drive means is then stabilized in inertial space using the output-of the inertial angular rotation sensor as a reference. Various modes of operation and control are provided for the drive means, and may include one or more of the following:
1. Stabilization directly to the inherent null output of the inertial angular rotation sensor;
2. Stabilization in any fixed position about the borehole axis using the inertial angular rotation sensor, but:
a) referenced to accelerometer data,
b) referenced to magnetometer data,
c) referenced to a rotation angle sensor provided as part of the rotary drive means;
3. Continuous or intermittent rotation of the sensing devices, but controlled accurately to any selected rate, or to any desired number of stopping points.
In such modes of operation, the primary stabilization reference is the inertial angular rotation sensor. The specific modes referred to above may be achieved by combining other data into the control means for the rotary drive, along with the output of the inertial angular rotation sensor. The inertial angular rotation sensor may be an inertial angular accelerometer, an inertial angular rate sensor or an inertial angle sensor.
In another alternative embodiment, no rotary drive means is provided. The output of the inertial angular rotation sensor is used to directly stabilize, by computation, the outputs of the cross-borehole magnetometer and accelerometer sensors into-an earth-fixed coordinate set.
These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: